The substantial and increasing encroachment of trade agreements into almost every aspect of economic and social life make it imperative for trade unions to engage in trade policy and agreement making processes, not just at the international but also at the national level. However, union staff frequently lack knowledge, not only about the substantive issues related to trade, but also about the trade policy process within their own country. This research project, initiated by the Global Labour University (GLU), is an attempt to address this gap by undertaking a comparative analysis of trade union capacity to influence trade policies at the national level. The analysis draws on case studies from a wide spectrum, ranging from developed and newly industrialised to developing countries. A framework drawn from social movement theory is used to reveal how differences in the political opportunity structure available within each country impacts on the strategic choices available to unions, and also, how trade policy processes and unions’ own organisational structures and dynamics impact on their capacity to mobilise resources for action against trade liberalisation. Great variation is found in the countries studied, both in terms of political opportunity structure and in terms of trade union capacity. In very few cases do unions have both institutional opportunity to intervene in trade policy processes and sufficiently developed policy expertise and mobilising capacity to take advantage of such opportunity. However, the research also shows that institutional opportunity is not static and unions can have an impact on trade policy and negotiations, even where they have limited access and capacity.